ABA Integers: the Undulating Numbers

In this challenge, you have to establish if a given number is undulating. A number n is undulating when the following conditions are all true:

• n has at least three digits.
• n has exactly two different digits.
• The two digits of n are alternating without repeating groups.

If we think of the first digit of an undulating number as an "A", and the second digit as a "B", we notice a sequence of the form "ABA" that can repeat infinite times and ends either with an "A" or with a "B", but without clusters of "AA" or "BB" in it.

Given a positive integer n, implement a function that returns True if n is an Undulating number, or False if it's not.

Examples

is_undulating(121) ➞ True
# A = 1, B = 2
# The sequence ABA is valid

is_undulating(373737) ➞ True
# A = 3, B = 7
# The sequence ABABABAB is valid

is_undulating(12) ➞ False
# Less than three digits

is_undulating(12122) ➞ False
# The sequence ABABB is not valid

is_undulating(85856) ➞ False
# More than two different digits

Notes

N/A